How to prove that
$$2\sin \frac{\pi}{14}+2\sin \frac{5\pi}{14}-2\sin \frac{3\pi}{14}=1$$
I absolutely have no idea other than using the property$\sin\bigl(\frac{\pi}{2}-a\bigr)=\cos a\;$
How to find $\lim_{h\rightarrow 0}\frac{\sin(ha)}{h}$ without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...
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